The present invention relates generally to extracorporeal systems for oxygenating and pumping blood during, for example, cardiac surgery. More specifically, the present invention relates to a method and system for improving gas exchange properties of oxygenators which utilize hollow fiber membranes for removing carbon dioxide and/or adding oxygen to a patient's blood via extracorporeal circulation.
It has been reported that 350,000 Americans die of lung disease each year, most from Acute Respiratory Distress Syndrome (ARDS) and Chronic Obstructive Pulmonary Disease (COPD). The most common treatment is mechanical ventilation, but may further exacerbate respiratory insufficiency and can cause serious side effects, such as barotrauma and volutrauma. It has been further reported that oxygenators are commonly utilized throughout the world in heart-lung machines, which are employed during surgery, and Extra Corporeal Membrane Oxygenation (ECMO) therapy, which is used to treat patients with compromised cardiopulmonary functions. Such oxygenators may be useful in treating COPD and ARDS. However, inefficient mass transfer (gas exchange) of oxygen and carbon dioxide is a common problem in oxygenators used in heart-lung machines and ECMO therapy.
The use of membrane oxygenators to oxygenate blood is well known in the art. Typically, they are disposable components which employ bundles of tiny hollow fibers made from a special polymer material having microscopic pores. The hollow fibers are generally impermeable to blood and permeable to gas. These fiber bundles are contained within a housing which includes an opening or port for receiving venous blood from a patient and an exit port through which the now oxygenated blood exits the oxygenator and is returned to the patient. Blood enters the oxygenator and flows around the outside surfaces of these fiber bundles. At the same time, a gas medium is pumped through the hollow fibers. This gas medium, often referred as a “sweep gas,” can be, for example, air, oxygen or an oxygen-rich gas which may also include an additive such as an anesthetic agent. Based on the law of diffusion, the oxygen contained in the sweep gas diffuses through the microscopic pores of the fibers to enrich the venous blood which contacts the outer surface of the hollow fibers. Due to the high concentration of carbon dioxide in the blood arriving from the patient, some carbon dioxide contained in the blood will likewise diffuse through the microscopic pores into the lumens of the fibers and into the sweep gas. The sweep gas is exhausted from the oxygenator after the oxygen enriching process takes place. As a result of this exchange, the oxygen content of the blood will be raised while the carbon dioxide level will be decreased. In some systems, the blood entering the oxygenator can be heated or cooled prior to be returned to the patient.
In the course of oxygenating blood utilizing a conventional oxygenator, water vapor from the patient's blood can permeate the hollow fiber membrane and condense in the membrane's micro-pores. This condensation effectively increases the diffusion length for the gas transfer and reduces gas exchange efficiency of the oxygenator. Concurrently, condensed water can accumulate in the lumens of the fibers and collect in the bottom of the fibers, which can substantially block or at least somewhat diminish the flow of the sweep gas through the fibers. As the number of blocked fibers increases, so too does the pressure drop across the fiber bundle. This accumulation of water in the fibers also affects carbon dioxide exchange because carbon dioxide can build up in the water barrier, thereby decreasing the driving concentration gradient needed for carbon dioxide transfer. When a sufficient number of fibers become blocked, the pressure drop across the bundle reaches that of the surface tension required to push the accumulated moisture from the fiber ends. As a result of this phenomenon, gas exchange gradually decreases until the number of blocked fibers increases to a level where the pressure drop across the oxygenator equals the droplet surface tension and equilibrium is achieved. This accumulation of moisture in the fibers is unwanted and will diminish the gas exchange efficiency of the oxygenator.
“Coughing” of the oxygenator has been described as a method to increase the instantaneous flow across the oxygenator (and associated pressure drop), thereby effecting a purge (removal of the accumulated moisture) similar to a cough in a patient. There are limitations to this method, however. Since the coughing of the oxygenators raises the pressure of the sweep gas compartment (i.e., the lumens of the hollow fiber), the risk of gas embolus forming in the blood and flowing back to the patient is dramatically increased. Accordingly, when the oxygenator is being coughed, the increased pressure of the sweep gas in the hollow fibers should never exceed the pressure in the blood compartment (typically below 200 mmHg). However, since positive pressure is utilized to increase the sweep gas pressure to generate the “cough,” it is often difficult, if not impossible, to prevent the sweep gas pressure from becoming higher than the pressure in the blood compartment of the oxygenator.
Accordingly, there is a need for, and what was heretofore unavailable, a method and system for enhancing the gas exchange characteristics of an oxygenator by overcoming the limitations and dangers associated with “coughing” an oxygenator utilizing conventional methods while providing a safe and reliable way to remove accumulated moisture from within the hollow fibers of the oxygenator. The present invention described herein satisfies these and other needs.